Answer:
Option third is True.
Explanation:
We have been given the exponential expression

The exponent 7/9 is with the variable m only, not on 3. Hence, option first is not correct.
Also rational exponent is 7/9. Hence, second option is also incorrect.
Now, we can rewrite the expressions as

Now, using the rule
![x^{(1)/(n)}=\sqrt[n]{x}](https://img.qammunity.org/2020/formulas/mathematics/college/e10pncyw3ldgevqk74jwva3rxxwv30ubqn.png)
![3m^{(7)/(9)}=\sqrt[9]{m^7}](https://img.qammunity.org/2020/formulas/mathematics/college/o7nlkjdq3c35aabt67v4d4r3b0wpfpc66v.png)
Hence, 9 is the index
Therefore, option third is True.